Olfactory receptors are encoded by the largest gene family in the mammalian genome. In most mammals there are over 1100 odor receptors (ORs) and 300 Vomeronasal pheromone receptors (VRs);even in humans there are nearly 1000 ORs although 65% are pseudogenes. Although the primary role of these receptors is to bind odorous ligands and initiate the signaling pathway in olfactory sensory neurons (OSNs), recently new functions for them have been defined. One of these is a critical role in axon guidance and targeting, insuring that all the OSNs expressing the same receptor send their axons to a restricted set of glomeruli in the olfactory bulb. A second is a possible role in feedback gene regulation that insures that each OSN expresses only one out of the hundreds of OR genes. This proposal seeks to investigate these new roles using multiple and new experimental strategies. In Aim 1 we will extend our recent bioinformatics work in mouse into comparative genomics, using the olfactory sub-genomes from rat, dog, human and chimp. These studies will make use not only of the new OR genes now sequenced in these species but also the inter-genie DNA that is likely to contain critical and conserved regulatory motifs. In a second major aim we will explore the targeting and regulatory functions of ORs by using a retroviral technique to drive expression of a second receptor and other genes in the signaling pathway in early progenitors of OSNs, enabling us to arrive at a deeper understanding of these OR functions by breaking the "one receptor- one neuron" rule. We will also perform these experiments in the complementary vomeronasal system. Odor receptors, aside from their importance to the sense of smell, are G-protein coupled types of receptors. GPCRs are a major target of drugs and treatments for many neural, psychological and other pathologies. A deeper understanding of ORs will lead to more fundamental insights into GPGRs.